For baseball pitchers, it's the earned run average. For National Football League passers, it's the quarterback rating. In each case, a single number gives some sense of a player's value.
How would you quantify the cumulative impact and relevance of a researcher's scientific work? Furthermore, is there a single number that would usefully characterize a scientist's output?
"In a world of limited resources, such quantification (even if potentially distasteful) is often needed for evaluation and comparison purposes," physicist Jorge E. Hirsch of the University of California, San Diego argues in the Nov. 15 Proceedings of the National Academy of Sciences.
Such ratings can be (and already have been) used to measure the impact of research and to justify tenure and funding decisions. Analyses of these measures can also generate rankings of scholarly journals, academic departm